The research, published in the journal BMJ Open, found that 42% of products surveyed contained at least 19g of sugars - a child's entire maximum recommended daily amount of sugars.

Experts from the University of Liverpool and Queen Mary University of London, examined the sugar content of all fruit juices, juice drinks and smoothies that are marketed to children.

After looking at both supermarket own and branded products, they found that 64% contained at least half of a young child's maximum sugar intake for the day.

Children aged four to six are recommended to have a maximum of 19g of sugar a day, they said.

The authors of the paper said that drinks with a high sugar content should not count as one the the Government's 5 a Day.

" Ideally, fruit should be consumed in its whole form, not as juice," they wrote.

"Parents should dilute fruit juice with water, opt for unsweetened juices and only give them during meals. Portions should be limited to 150ml a day.

"In order to help combat the growing problem of childhood obesity, manufacturers need to stop adding unnecessary sugars and calories to their fruit juices, juice drinks and smoothies now; otherwise, it will be essential for the Government to introduce legislation to regulate the free sugars content of these products."

They added: "Current guidelines state a 150ml glass of pure fruit juice at meal times counts as a maximum of one of the '5 a Day'.

"However, Public Health England (PHE) is aiming to refresh the '5 a Day' campaign, including a reconsideration of the advice on fruit juice and smoothies. This appears crucial, based on our findings."

But Dr Louis Levy, head of nutrition science at PHE, said: "We know that juice and smoothies are high in sugar which is why PHE recommends limiting them to a combined total of 150ml per day, to be drunk with a meal to protect your teeth.

"However they also provide some fibre, vitamins and minerals so count towards one of your 5 A Day. The other recommendations for individuals in this paper support current Government advice."

The researchers identified 203 fruit juices, juice drinks and smoothies marketed to children across seven major UK supermarkets.

The mean sugar content was 7.0 g per 100 ml, but among the 100% fruit juice category, it was 10.7 g per 100 ml.

They found that 117 of the 203 drinks surveyed would receive a Food Standards Agency 'red' colour-coded label for sugars per serving.

As part of his Budget, Chancellor George Osborne announced a new sugar levy on the soft drinks industry. But t he new sugar tax on soft drinks will not be paid on milk-based drinks and fruit juices, the Treasury has said.

Commenting on the study, Dr Gunter Kuhnle, food scientist at the University of Reading, added: "T hese drinks, fruit juices, fruit drinks and smoothies are often seen as a 'healthy' alternative and their sugar content is ignored. This study shows that the average sugar content in fruit juices is similar to that in cola drinks; and the content in smoothies is even higher, by almost three sugar cubes per 300ml serving."